CASE
1
Eluding Aeolus: Turbulence,Gusts, and Wind Shear
Since the earliest days of American aeronautical research, NASA has studied the atmosphere and its influence upon flight. Turbulence, gusts, and wind shears have posed serious dangers to air travelers, forcing imaginative research and creative solutions. The work of NASA’s researchers to understand atmospheric behavior and NASA’s derivation of advanced detection and sensor systems that can be installed in aircraft have materially advanced the safety and utility of air transport.
Case-1 Cover Image: NASA 515, Langley Research Center’s Boeing 737 testbed, is about to enter a microburst wind shear. The image is actual test footage, reflecting the murk and menace of wind shear. NASA.
Before World War II, the National Advisory Committee for Aeronautics (NACA), founded in 1915, performed most of America’s institutionalized and systematic aviation research. The NACA’s mission was “to supervise and direct the scientific study of the problems of flight with a view to their practical solution.” Among the most serious problem it studied was that of atmospheric turbulence, a field related to the Agency’s great interest in fluid mechanics and aerodynamics in general. From the 1930s to the present, the NACA and its successor—the National Aeronautics and Space Administration (NASA), formed in 1958—concentrated rigorously on the problems of turbulence, gusts, and wind shear. Midcentury programs focused primarily on gust load and boundary-layer turbulence research. By the 1980s and 1990s, NASA’s atmospheric turbulence and wind shear programs reached a level of sophistication that allowed them to make significant contributions to flight performance and aircraft reliability. The aviation industry integrated this NASA technology into planes bought by airlines and the United States military. This research has resulted in an aviation transportation system exponentially safer than that envisioned by the pioneers of the early air age.
An Unsettled Sky
When laypeople think of the words “turbulence” and “aviation” together, they probably envision the “bumpy air” that passengers are often subjected to on long-duration plane flights. But the term “turbulence” has a particular technical meaning. Turbulence describes the motion of a fluid (for, our purposes, air) that is characterized by chaotic, seemingly random property changes. Turbulence encompasses fluctuations in diffusion, convection, pressure, and velocity. When an aircraft travels through air that experiences these changes, its passengers feel the turbulence buffeting the aircraft. Engineers and scientists characterize the degree of turbulence with the Reynolds number, a scaling parameter identified in the 1880s by Osborne Reynolds at the University of Manchester. Lower numbers denote laminar (smooth) flows, intermediate values indicate transitional flows, and higher numbers are characteristic of turbulent flow.[1]
A kind of turbulent airflow causes drag on all objects, including cars, golf balls, and planes, which move through the air. A boundary layer is “the thin reaction zone between an airplane [or missile] and its external environment.” The boundary layer is separated from the contour of a plane’s airfoil, or wing section, by only a few thousandths of an inch. Air particles change from a smooth laminar flow near the leading edge to a turbulent flow toward the airfoil’s rear.[2] Turbulent flow increases friction on an aircraft’s skin and therefore increased surface heat while slowing the speed of the aircraft because of the drag it produces.
Most atmospheric circulation on Earth causes some kind of turbulence. One of the more common forms of atmospheric turbulence experienced by aircraft passengers is clear air turbulence (CAT), which is caused by the mixing of warm and cold air in the atmosphere by wind, often via the process of wind shear. Wind shear is a difference in wind speed and direction over a relatively short distance in Earth’s atmosphere. One engineer describes it as “any situation where wind velocity varies sharply from point to point.”[3] Wind shears can have both horizontal and vertical components. Horizontal wind shear is usually encountered near coastlines and along fronts, while vertical wind shear appears closer to Earth’s surface and sometimes at higher levels in the atmosphere, near frontal zones and upper-level air jets.
Large-scale weather events, such as weather fronts, often cause wind shear. Weather fronts are boundaries between two masses of air that have different properties, such as density, temperature, or moisture. These fronts cause most significant weather changes. Substantial wind shear is observed when the temperature difference across the front is 9 degrees Fahrenheit (ºF) or more and the front is moving at 30 knots or faster. Frontal shear is seen both vertically and horizontally and can occur at any altitude between surface and tropopause, which is the lowest portion of Earth’s atmosphere and contains 75 percent of the atmosphere’s mass. Those who study the effects of weather on aviation are concerned more with vertical wind shear above warm fronts than behind cold fronts because of the longer duration of warm fronts.[4]
The occurrence of wind shear is a microscale meteorological phenomenon. This means that it usually develops over a distance of less than 1 kilometer, even though it can emerge in the presence of large weather patterns (such as cold fronts and squall lines). Wind shear affects the movement of soundwaves through the atmosphere by bending the wave front, causing sounds to be heard where they normally would not. A much more violent variety of wind shear can appear near and within downbursts and microbursts, which may be caused by thunderstorms or weather fronts, particularly when such phenomena occur near mountains. Vertical shear can form on the lee side of mountains when winds blow over them. If the wind flow is strong enough, turbulent eddies known as “rotors” may form. Such rotors pose dangers to both ascending and descending aircraft.[5]
The microburst phenomenon, discovered and identified in the late 1970s by T. Theodore Fujita of the University of Chicago, involves highly localized, short-lived vertical downdrafts of dense cool air that impact the ground and radiate outward toward all points of the compass at high speed, like a water stream from a kitchen faucet impacting a basin.[6]
Speed and directional wind shear result at the three-dimensional boundary’s leading edge. The strength of the vertical wind shear is directly proportional to the strength of the outflow boundary. Typically, microbursts are smaller than 3 miles across and last fewer than 15 minutes, with rapidly fluctuating wind velocity.[7]
Wind shear is also observed near radiation inversions (also called nocturnal inversions), which form during rapid cooling of Earth’s surface at night. Such inversions do not usually extend above the lower few hundred feet in the atmosphere. Favorable conditions for this type of inversion include long nights, clear skies, dry air, little or no wind, and cold or snow-covered surfaces. The difference between the inversion layer and the air above the inversion layer can be up to 90 degrees in direction and 40 knots. It can occur overnight or the following morning. These differences tend to be strongest toward sunrise.[8]
The troposphere is the lowest layer of the atmosphere in which weather changes occur. Within it, intense vertical wind shear can slow or prevent tropical cyclone development. However, it can also coax thunderstorms into longer life cycles, worsening severe weather.[9]
Wind shear particularly endangers aircraft during takeoff and landing, when the aircraft are at low speed and low altitude, and particularly susceptible to loss of control. Microburst wind shear typically occurs during thunderstorms but occasionally arises in the absence of rain near the ground. There are both “wet” and “dry” microbursts. Before the developing of forward-looking detection and evasion strategies, it was a major cause of aircraft accidents, claiming 26 aircraft and 626 lives, with over 200 injured, between 1964 and 1985.[10]
Another macro-level weather event associated with wind shear is an upper-level jetstream, which contains vertical and horizontal wind shear at its edges. Jetstreams are fast-flowing, narrow air currents found at certain areas of the tropopause. The tropopause is the transition between the troposphere (the area in the atmosphere where most weather changes occur and temperature decreases with height) and the stratosphere (the area where temperature increases with height).[11] A combination of atmospheric heating (by solar radiation or internal planetary heat) and the planet’s rotation on its axis causes jetstreams to form. The strongest jetstreams on Earth are the polar jets (23,000–39,000 feet above sea level) and the higher and somewhat weaker subtropical jets (33,000–52,000 feet). Both the northern and southern hemispheres have a polar jet and a subtropical jet. Wind shear in the upper-level jetstream causes clear air turbulence. The cold-air side of the jet, next to the jet’s axis, is where CAT is usually strongest.[12]
Although most aircraft passengers experience clear air turbulence as a minor annoyance, this kind of turbulence can be quite hazardous to aircraft when it becomes severe. It has caused fatalities, as in the case of United Airlines Flight 826.[13] Flight 826 took off from Narita International Airport in Japan for Honolulu, HI, on December 28, 1997.
At 31,000 feet, 2 hours into the flight, the crew of the plane, a Boeing 747, received warning of severe clear air turbulence in the area. A few minutes later, the plane abruptly dropped 100 feet, injuring many passengers and forcing an emergency return to Tokyo, where one passenger subsequently died of her injuries.[14] A low-level jetstream is yet another phenomenon causing wind shear. This kind of jetstream usually forms at night, directly above Earth’s surface, ahead of a cold front. Low-level vertical wind shear develops in the lower part of the low-level jet. This kind of wind shear is also known as nonconvective wind shear, because it is not caused by thunderstorms.
The term “jetstream” is often used without further modification to describe Earth’s Northern Hemisphere polar jet. This is the jet most important for meteorology and aviation, because it covers much of North America, Europe, and Asia, particularly in winter. The Southern Hemisphere polar jet, on the other hand, circles Antarctica year-round.[15] Commercial use of the Northern Hemisphere polar jet began November 18, 1952, when a Boeing 377 Stratocruiser of Pan American Airlines first flew from Tokyo to Honolulu at an altitude of 25,000 feet. It cut the trip time by over one-third, from 18 to 11.5 hours.[16] The jetstream saves fuel by shortening flight duration, since an airplane flying at high altitude can attain higher speeds because it is passing through less-dense air. Over North America, the time needed to fly east across the continent can be decreased by about 30 minutes if an airplane can fly with the jetstream but can increase by more than 30 minutes it must fly against the jetstream.[17]
Strong gusts of wind are another natural phenomenon affecting aviation. The National Weather Service reports gusts when top wind speed reaches 16 knots and the variation between peaks and lulls reaches 9 knots.[18] A gust load is the wind load on a surface caused by gusts.
Otto Lilienthal, the greatest of pre-Wright flight researchers, in flight. National Air and Space Museum.
The more physically fragile a surface, the more danger a gust load will pose. As well, gusts can have an upsetting effect upon the aircraft’s flightpath and attitude.
Initial NACA–NASA Research
Sudden gusts and their effects upon aircraft have posed a danger to the aviator since the dawn of flight. Otto Lilienthal, the inventor of the hang glider and arguably the most significant aeronautical researcher before the Wright brothers, sustained fatal injuries in an 1896 accident, when a gust lifted his glider skyward, died away, and left him hanging in a stalled flight condition. He plunged to Earth, dying the next day, his last words reputedly being “Opfer müssen gebracht werden”—or “Sacrifices must be made.”[19]
NASA’s interest in gust and turbulence research can be traced to the earliest days of its predecessor, the NACA. Indeed, the first NACA technical report, issued in 1917, examined the behavior of aircraft in gusts.[20] Over the first decades of flight, the NACA expanded its interest in gust research, looking at the problems of both aircraft and lighter-than-air airships. The latter had profound problems with atmospheric turbulence and instability: the airship Shenandoah was torn apart over Ohio by violent stormwinds; the Akron was plunged into the Atlantic, possibly from what would now be considered a microburst; and the Macon was doomed when clear air turbulence ripped off a vertical fin and opened its gas cells to the atmosphere. Dozens of airmen lost their lives in these disasters.[21]
During the early part of the interwar years, much research on turbulence and wind behavior was undertaken in Germany, in conjunction with the development of soaring, and the long-distance and long-endurance sailplane. Conceived as a means of preserving German aeronautical skills and interest in the wake of the Treaty of Versailles, soaring evolved as both a means of flight and a means to study atmospheric behavior. No airman was closer to the weather, or more dependent upon an understanding of its intricacies, than the pilot of a sailplane, borne aloft only by thermals and the lift of its broad wings. German soaring was always closely tied to the nation’s excellent technical institutes and the prestigious aerodynamics research of Ludwig Prandtl and the Prandtl school at Göttingen. Prandtl himself studied thermals, publishing a research paper on vertical air currents in 1921, in the earliest years of soaring development.[22] One of the key figures in German sailplane development was Dr. Walter Georgii, a wartime meteorologist who headed the postwar German Research Establishment for Soaring Flight (Deutsche Forschungsanstalt für Segelflug ([DFS]). Speaking before Britain’s Royal Aeronautical Society, he proclaimed, “Just as the master of a great liner must serve an apprenticeship in sail craft to learn the secret of sea and wind, so should the air transport pilot practice soaring flights to gain wider knowledge of air currents, to avoid their dangers and adapt them to his service.”[23] His DFS championed weather research, and out of German soaring, came such concepts as thermal flying and wave flying. Soaring pilot Max Kegel discovered firsthand the power of storm-generated wind currents in 1926. They caused his sailplane to rise like “a piece of paper that was being sucked up a chimney,” carrying him almost 35 miles before he could land safely.[24] Used discerningly, thermals transformed powered flight from gliding to soaring. Pioneers such as Gunter Grönhoff, Wolf Hirth, and Robert Kronfeld set notable records using combinations of ridge lift and thermals. On July 30, 1929, the courageous Grönhoff deliberately flew a sailplane with a barograph into a storm, to measure its turbulence; this flight anticipated much more extensive research that has continued in various nations.[25]
The NACA first began to look at thunderstorms in the 1930s. During that decade, the Agency’s flagship laboratory—the Langley Memorial Aeronautical Laboratory in Hampton, VA—performed a series of tests to determine the nature and magnitude of gust loadings that occur in storm systems. The results of these tests, which engineers performed in Langley’s signature wind tunnels, helped to improve both civilian and military aircraft.[26] But wind tunnels had various limitations, leading to use of specially instrumented research airplanes to effectively use the sky as a laboratory and acquire information unobtainable by traditional tunnel research. This process, most notably associated with the post–World War II X-series of research airplanes, led in time to such future NASA research aircraft as the Boeing 737 “flying laboratory” to study wind shear. Over subsequent decades, the NACA’s successor, NASA, would perform much work to help planes withstand turbulence, wind shear, and gust loadings.
From the 1930s to the 1950s, one of the NACA’s major areas of research was the nature of the boundary layer and the transition from laminar to turbulent flow around an aircraft. But Langley Laboratory also looked at turbulence more broadly, to include gust research and meteorological turbulence influences upon an aircraft in flight. During the previous decade, experimenters had collected measurements of pressure distribution in wind tunnels and flight, but not until the early 1930s did the NACA begin a systematic program to generate data that could be applied by industry to aircraft design, forming a committee to oversee loads research. Eventually, in the late 1930s, Langley created a separate structures research division with a structures research laboratory. By this time, individuals such as Philip Donely, Walter Walker, and Richard V. Rhode had already undertaken wideranging and influential research on flight loads that transformed understanding about the forces acting on aircraft in flight. Rhode, of Langley, won the Wright Brothers Medal in 1935 for his research of gust loads. He pioneered the undertaking of detailed assessments of the maneuvering loads encountered by an airplane in flight. As noted by aerospace historian James Hansen, his concept of the “sharp edge gust” revised previous thinking of gust behavior and the dangers it posed, and it became “the backbone for all gust research.”[27] NACA gust loads research influenced the development of both military and civilian aircraft, as did its research on aerodynamic-induced flight-surface flutter, a problem of particular concern as aircraft design transformed from the era of the biplane to that of the monoplane. The NACA also investigated the loads and stresses experienced by combat aircraft when undertaking abrupt rolling and pullout maneuvers, such as routinely occurred in aerial dogfighting and in dive-bombing.[28] A dive bomber encountered particularly punishing aerodynamic and structural loads as the pilot executed a pullout: abruptly recovering the airplane from a dive and resulting in it swooping back into the sky. Researchers developed charts showing the relationships between dive angle, speed, and the angle required for recovery. In 1935, the Navy used these charts to establish design requirements for its dive bombers. The loads program gave the American aeronautics community a much better understanding of load distributions between the wing, fuselage, and tail surfaces of aircraft, including high-performance aircraft, and showed how different extreme maneuvers “loaded” these individual surfaces.
In his 1939 Wilbur Wright lecture, George W. Lewis, the NACA’s legendary Director of Aeronautical Research, enumerated three major questions he believed researchers needed to address:
Answering these questions, posed at the close of the biplane era, would consume researchers for much of the next six decades, well into the era of jet airliners and supersonic flight.
The advent of the internally braced monoplane accelerated interest in gust research. The long, increasingly thin, and otherwise unsupported cantilever wing was susceptible to load-induced failure if not well-designed. Thus, the stresses caused by wind gusts became an essential factor in aircraft design, particularly for civilian aircraft. Building on this concern, in 1943, Philip Donely and a group of NACA researchers began design of a gust tunnel at Langley to examine aircraft loads produced by atmospheric turbulence and other unpredictable flow phenomena and to develop devices that would alleviate gusts. The tunnel opened in August 1945. It utilized a jet of air for gust simulation, a catapult for launching scaled models into steady flight, curtains for catching the model after its flight through the gust, and instruments for recording the model’s responses. For several years, the gust tunnel was useful, “often [revealing] values that were not found by the best known methods of calculation . . . in one instance, for example, the gust tunnel tests showed that it would be safe to design the airplane for load increments 17 to 22 percent less than the previously accepted values.”[30] As well, gust researchers took to the air. Civilian aircraft—such as the Aeronca C-2 light, general-aviation airplane, Martin M-130 flying boat, and the Douglas DC-2 airliner—and military aircraft, such as the Boeing XB-15 experimental bomber, were outfitted with special loads recorders (so-called “v-g recorders,” developed by the NACA). Extensive records were made on the weather-induced loads they experienced over various domestic and international air routes.[31]
The experimental Boeing XB-15 bomber was instrumented by the NACA to acquire gust-induced structural loads data. NASA.
This work was refined in the postwar era, when new generations of long-range aircraft entered air transport service and were also instrumented to record the loads they experienced during routine airline operation.[32] Gust load effects likewise constituted a major aspect of early transonic and supersonic aircraft testing, for the high loads involved in transiting from subsonic to supersonic speeds already posed a serious challenge to aircraft designers. Any additional loading, whether from a wind gust or shear, or from the blast of a weapon (such as the overpressure blast wave of an atomic weapon), could easily prove fatal to an already highly loaded aircraft.[33] The advent of the long-range jet bomber and transport—a configuration typically having a long and relatively thin swept wing, and large, thin vertical and horizontal tail surfaces—added further complications to gust research, particularly because the penalty for an abrupt gust loading could be a fatal structural failure. Indeed, on one occasion, while flying through gusty air at low altitude, a Boeing B-52 lost much of its vertical fin, though fortunately, its crew was able to recover and land the large bomber.[34]
The emergence of long-endurance, high-altitude reconnaissance aircraft such as the Lockheed U-2 and Martin RB-57D in the 1950s and the long-range ballistic missile further stimulated research on high-altitude gusts and turbulence. Though seemingly unconnected, both the high-altitude jet airplane and the rocket-boosted ballistic missile required understanding of the nature of upper atmosphere turbulence and gusts. Both transited the upper atmospheric region: the airplane cruising in the high stratosphere for hours, and the ballistic missile or space launch vehicle transiting through it within seconds on its way into space. Accordingly, from early 1956 through December 1959, the NACA, in cooperation with the Air Weather Service of the U.S. Air Force, installed gust load recorders on Lockheed U-2 strategic reconnaissance aircraft operating from various domestic and overseas locations, acquiring turbulence data from 20,000 to 75,000 feet over much of the Northern Hemisphere. Researchers concluded that the turbulence problem would not be as severe as previous estimates and high-altitude balloon studies had indicated.[35]
High-altitude loitering aircraft such as the U-2 and RB-57 were followed by high-altitude, high-Mach supersonic cruise aircraft in the early to mid-1960s, typified by Lockheed’s YF-12A Blackbird and North American’s XB-70A Valkyrie, both used by NASA as Mach 3+ Supersonic Transport (SST) surrogates and supersonic cruise research testbeds. Test crews found their encounters with high-altitude gusts at supersonic speeds more objectionable than their exposure to low-altitude gusts at subsonic speeds, even though the given g-loading accelerations caused by gusts were less than those experienced on conventional jet airliners.[36] At the other extreme of aircraft performance, in 1961, the Federal Aviation Agency (FAA) requested NASA assistance to document the gust and maneuver loads and performance of general-aviation aircraft. Until the program was terminated in 1982, over 35,000 flight-hours of data were assembled from 95 airplanes, representing every category of general-aviation airplane, from single-engine personal craft to twin-engine business airplanes and including such specialized types as crop-dusters and aerobatic aircraft.[37]
Along with studies of the upper atmosphere by direct measurement came studies on how to improve turbulence detection and avoidance, and how to measure and simulate the fury of turbulent storms. In 1946–1947, the U.S. Weather Bureau sponsored a study of turbulence as part of a thunderstorm study project. Out of this effort, in 1948, researchers from the NACA and elsewhere concluded that ground radar, if properly used, could detect storms, enabling aircraft to avoid them. Weather radar became a common feature of airliners, their once-metal nose caps replaced by distinctive black radomes.[38] By the late 1970s, most wind shear research was being done by specialists in atmospheric science, geophysical scientists, and those in the emerging field of mesometeorology—the study of s